Why am I missing nodes in the 'Add node' menu?
There are three possible causes;


1- The render engine might set to Blender Render (also known as Blender Internal) instead of Cycles. Materials work very differently in each of the render engines, and are not interchangeable.



Blender internal does not use shaders, Cycles does.





To learn more about the difference between render engines read:How is Cycles different from Blender Internal? and Cycles vs internal vs game engine
2- The node editor might be set to a different mode.

There are three different modes for the node editor and they use different kinds of nodes.



By default the node editor is configured for compositing .

Change to the Material node editor (and make sure that the Use Nodes box is enabled).

:



More info: 

https://www.blender.org/manual/editors/node_editor/introduction.html#node-tree-types

https://www.blender.org/manual/editors/node_editor/editor.html
3- If Cycles is not available in the render engine list, then it needs to be enabled in the System Preferences>Addon List (Access the system Preferences window by pressing CtrlAltU)

Look for the Cycles Addon in the render section, enable it using the check box and then save the user settings to enable cycles permanently.